My favorite combo is good ol' Myers Dark and Kaniche Rhum Martinique. That said, Kaniche is an almost-impossible to find brand these days, although I've managed to stock up on a supply that should last me a couple of years.

If you go back to Vic's very first Mai Tai recipe, it calls for 1-ounce 15-year old J. Wray Nephew Jamaican Rum and 1 ounce Coruba Jamaican Rum. He later switched to Martinique rum because he couldn't get a supply of the J. Wray and Nephew Rum anymore. Now, if you look at what's available these days, you'll see that Appleton Rum is made by J. Wray Nephew. So I figured that Appleton Estate Extra might be pretty close to what Vic used, since it's also aged up to 18-years (although it is a blend.) I tried it, along with the Coruba, and it was excellent! I would recommend you give it a whirl...

Myers's Dark Jamican & Mt. Gay Barbados Sugar Cane make the perfect Mai Tai rums, although at times I'll replace the Mt. Gay with St. James Hors d'Age (Martinique) for some extra richness.
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I hate to sound cheezy, but Trader Vic's Mai Tai rum makes a rather nice Mai Tai. It has such a unique aroma and taste that I haven't been able to duplicate with any blend of other rums. That, and it is only about $8 a bottle.